Apparatus for assembling a print head

ABSTRACT

A method and jig for assembling a plurality of thermal print wafers to form a print head. The jig includes a fixture plate having accurate first and second planar surfaces perpendicular to each and joined in a common edge. An alignment bar having first and second control surfaces thereon is secured to said plate so that the control surfaces lie in a common plane which is perpendicular to the first planar surface and parallel to the common edge. The print wafers are sandwiched between clamps and positioned on the jig so that the operative ends of the wafers become aligned in a plane controlled by the control surfaces when spring-loaded levers on the jig are used to urge the operative ends against the control surfaces. A compressing lever on the jig is used to compress the wafers between the clamps prior to securing the clamps together to produce a completed print wafer module. The completed print wafer modules are then aligned on a support member while using an alignment fixture having a plurality of control surfaces lying in a common plane. The operative end of each print wafer module is urged into contact with an adjacent pair of control surfaces on the alignment fixture to align the module, and the module is then secured to the support member. The remaining modules are similarly aligned and secured to the support member to produce a completed print head.

United States Patent [191 Mitchner et al.

1451 Nov. 27, 1973 APPARATUS FOR ASSEMBLING A PRINT HEAD [75] Inventors: Joseph L. Mitchner, Wilmington,

Ohio; Robert A. Keller, Seminole; Cylde B. Graham, St. Petersburg, both of Fla.

[73] Assignee: The National Cash Register Company, Dayton, Ohio [22] Filed: Mar. 20, 1972 [21] Appl. No.: 236,041

52 us. or. 269/315, 269/319 51 1m. (:1. B23q 3/00 [58] Field 6: Search... 29/464, 469, 203 P,

Primary Examiner Richard J. Herbst Assistant Examiner-James R. Duzan Attorney-J. T. Cavender et al.

l 1llll [5 7 ABSTRACT A method and jig for assembling a plurality of thermal print wafers to form a print head. The jig includes a fixture plate having accurate first and second planar surfaces perpendicular to each and joined in a common edge. An alignment bar having first and second control surfaces thereon is secured to said plate so that the control surfaces lie in a common plane which is perpendicular to the first planar surface and parallel to the common edge. The print wafers are sandwiched between clamps and positioned on the jig so that the operative ends of the wafers become aligned in a plane controlled by the control surfaces when springloaded levers on the jig are .used to urge the operative ends against the control surfaces. A compressing lever on the jig" is used to compress the wafers between the clamps prior to securing the clamps together to produce a completed print wafer module. The completed print wafer modules are then aligned on a support member while using an alignment fixture having a plurality of control surfaces lying in a common plane. The operative end of each print wafer module is urged into contact with an adjacent pair of control surfaces on the alignment fixture to align the module, and the module is then secured to the support member. The remaining modules are similarly aligned and secured to the support member to produce a completed print head.

3 Claims, 6 Drawing Figures PATENTED NOV 27 :915

SHEET 2 BF 2 FIG. 4

1 APPARATUS FOR ASSEMBLING A PRINT HEAD BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for assembling a plurality of print wafers to form a print head. This invention is especially adaptable for use in forming a thermal print head. A thermal print head generally has a plurality of tiny resistor elements formed on a substrate and arranged in a matrix (like a five by seven matrix) for producing a plurality of character patterns. The resistor elements are generally located in a common plane, and when in use, the resistor elements are brought into contact with a heat sensitive paper. Printing is effected by energizing selected ones of the resistor elements according to the character to be printed. When energized, the resistor elements heat up to a predetermined level causing the heat sensitive paper in contact therewith to change color, thereby effecting printing of the desired character pattern.

In one type of thermal print head, a plurality of individual resistor elements are formed on an operative end or edge of a print wafer made of a dielectric like a ceramic substrate which is sold under the trademark Fotoceram owned by Owens-Corning Glass Co. The print wafer additionally supports the connecting conductors associated with the elements. Several of these print wafers are stacked one on top of another to form a matrix for producing a character pattern. The print wafers are then sandwiched between two clamps which are used to secure the print wafers together so that all their operative ends lie in a common plane thereby producing a print wafer module. Several print wafer modules may be aligned to form a print head capable of printing a line of characters.

One of the problems with a print head of the type described in the previous paragraph is that unless all the operative ends of print wafers are accurately aligned in a common plane, the quality of the resulting printing produced thereby is poor. Various stacking and holding fxtures were tried in an effort to align the print head wafers and modules; however, none of them produced the accurate results achieved by the method and apparatusof the present invention.

For military applications of a thermal print head of the type described, it is necessary to provide an apparatus which can be used in the field or at depots for easily assembling the print wafers into modules and for assembling a plurality of modules into a print head for line printing. Completed print heads on printers may be removed therefrom, repaired, and reinstalled in the printers with ease and precision through using this invention. i

Some related print heads and print modules are shown in the following U.S. patents: U.S. Pat. No. 3,267,485 which issued on Aug. 16, 1966, and U.S. Pat. No. 3,478,191 which issued on Nov. 11, 1969.

SUMMARY OF THE INVENTION This invention relates to a method andapparatus for assembling several print wafers into a module and for assembling several modules into a print head.

The apparatus includes a jig and an alignment fixture. The jig includes a base and a fixture plate secured thereto. The fixture plate has accurate first and second planar surfaces which are perpendicular to each other and are joined along a common edge. An alignment bar having first and second control surfaces thereon is secured to the fixture plate so that the control surfaces lie in a common plane which is perpendicular to the first planar surface and parallel to the common edge. The print wafers are sandwiched between clamps and positioned on the jig so that the operative ends of the wafers become aligned in a plane controlled by the control surfaces when spring loaded first lever means are used to urge the operative ends against the controlsurfaces. A compressing lever means on the jig is used to compress the wafers between the clamps prior to securing the clamps together to produce a completed print wafer module. The completed print wafer modules are then aligned on a support member while using an alignment fixture having a plurality of control surfaces lying in a common plane. The operative end of each print wafer module is urged into contact with an adjacent pair of control surfaces on the alignment fixture to align the module, and the module isthen secured to the support member. The remaining modules are similarly aligned and secured to the support member to produce a completed print head.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of'a jig used in this invention to FIG. 2 is a side view, in elevation, of the jig shown in FIG. 1, showing additional details of alever means for compressing the print wafers.

FIG. 3 is an enlarged, plan view of a portion of FIG. 1, showing additional details of the control surfaces used in aligning the print wafers.

FIG. 4 is a general, exploded view, inperspective, of a plurality of print modules loosely secured to a support member (from a line printer) prior to detachablysecuh ing the support member to an alignment fixture for aligning the print wafer modules.

FIG. 5 is a plan view, in elevation, of the alignment fixture, which view is taken from the direction A of FIG. 4.

FIG. 6 is a corss-sectional view, taken alongthe line.

6- -6 of FIG. 5, showing additional'details of control surfaces located on the alignment fixture and how a print wafer module is alignedthereby.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a planview ofajig 10 used in this invention to align a plurality of print wafersto form aprint wafer within 0.0005 inch (flatness). An alignment bar 24, having first and second control surfaces 26 and 28, respectively, thereon is detachably secured to the front planar surface 22 by fasteners 30. The control surfaces 26 and 28 lie in a common plane which is perpendicular to the top surface 20 and is parallel to the common edge joining the surface 20 and 22. In the embodiment shown, the surfaces 26 and 28 are made of 300 Series stainless corrosion resistant steel. It is against these control surfaces 26 and 28 that the print wafers will be aligned. An alignment pin 32 is positioned near the common edge and the control surface 26 is secured to the fixture plate 14 so that its axis is perpendicular to the planar surface 20.

The jig also includes a first lever means 34 (FIG. 1) for contacting the rear ends of the print wafers to urge the operative or front ends of the wafers into contact with the first and second control surfaces. The lever means 34 includes a generally L shaped lever 36 which is pivotally mounted on a fastener 38 secured perpendicularly to the fixture plate 14 and another lever 40 which is also pivotally mounted on a'fastener '42 which is secured to the fixture plate 14. The lever 36 has a pad area 44 for contacting the rear ends of the print wafers at one side, and similarly, lever 40 has a pad area 46 for contacting the rear ends of the print wafers at the opposite side. Both areas 44 and 46 are planar surfaces which are perpendicular to the top surface and are made of silicon rubber. Both levers 36 and 40 are biased to rotate in a clockwise direction (as viewed in FIG. 1) by associated tension springs 48 and 50. A bar 52'is secured to the fixture plate 14 and is threaded to receive the adjustment screws 54 and 56. When screw 54 is advanced towards lever 36, it rotates lever 36 in a counterclockwise direction to force area 44 thereof into engagement with the rear ends of the print wafers. Similarly, when screw 56 is advanced towards lever 40, it rotates lever 40 in a counterclockwise direction to force area 46 thereofinto engagement with the rear ends of the print wafers.

The jig 10 also includes a second lever means 58 (shown in FIGS. 1 and 2) which is used to compress the print wafers while they are positioned on the jig. The lever means 58 includes a frame 60 which is secured to the base 12 as shown. A lever 62 has one end pivotally mounted to the frame 60 by a pin 64 and the remaining end is threaded to receive an adjustable stop 66 with locking nuts 68 thereon. An operating lever 70 is pivotally joined to one end of lever 62 by a pin 72, and it is pivotally joined between its ends to one end of link 74 by a pin 76. The remaining end of link 74 is pivotally joined to the frame 60 by a pin 78. When lever 70 is moved towards the base 12, the lever 62 is rotated in a counterclockwise direction (as viewed in FIG. 2) to bring the adjustable stop 66 to the position shown where it is used to compress the print wafers as will be later described herein. When the lever 70 is moved towards the base 12, it becomes locked, conventionally, in an above the center position to enable the stop 66 to continually apply a pressure on the print wafers until the lever 70 is moved away from the base 12.

The print wafers shown in the drawings are thermal print wafers. Each wafer, like 80 in FIG. 4, is made of Fotoceram material and has an operative end 82 on which the tiny resistive elements are located. Each wafer 80 has a flexible member 84 which is secured thereto and supports the conducting elements (not shown) which elements are connected to the resistive elements, and the conducting elements terminate in a isolation diode board 86. Because these aspects of the thermal print wafer may be conventional, they are not shown in detail.

The following method is used to form a print wafer module with the jig 10 shown in FIGS. 1, 2, and 3. The print wafers are sandwiched between upper and lower clamps, so the first step in the method of assembling the module is to detachably secure a lower clamp 88 to the fixture plate by fasteners 90 passing through a hole 92 formed in the base 12 and a portion of the plate 14 as shown in FIG. 2. The next step is to detachably secure the alignment bar 24 to the fixture plate 14. An insulating spacer (like 94 shown in FIG. 4) is then placed on top of the clamp 88, and a first print wafer like 80 is positioned on top of the spacer so that its associated flexible member 84 and connection strip 86 assume the position shown in dashed outline in FIG. 1. This positioning will bring the operative end 82 of the print wafer into contact with the first and second control surfaces 26 and 28. Another spacer like 94 is positioned on top of the first print wafer installed in the jig 10, and a second print wafer is positioned thereover. This process is repeated until five print wafers are installed in the jig (for a five by five print matrix) and the fifth or top print wafer is covered by another insulating spacer, like 96 shown in FIG. 4. An upper clamp 98 is positioned on top of the spacer 96, and fasteners 100 are inserted in the clamps to loosely hold them together. The fasteners 100 pass through notched-out areas 102 (FIG. 3) of the print wafers and insulating spacers. During the time that the print wafers are being installed in the jig l0, the springs 48 and 50 keep their associated levers out of contact with the rear ends of the wafers. When screw 54 (FIG. 1) is advanced, area 44 on lever 36 contacts the rear ends 104 of the print wafers to urge the operative ends thereof against the control surface 28. Similarly, when screw 56 is rotated to advance it against the lever 40, the area 46 of lever 40 contacts the rear ends 104 of the print wafers to urge the operative ends thereof against the control surface 26. Lever 40 lies close to the surface 20 and lies under the flexible members 84 associated with the print wafers; however, the lever, at section 106 (FIG. 1) thereof, is angled upwardly away from the surface 20 of fixture plate 14 to enable the associated area 46 to contact the rear ends 104 of the print wafers at the same level as area 44 as shown in FIG. 2. The print wafers are also pushed (in the direction of arrow 108 in FIG. 1) against the pin 32 with'a simple square edged wooden pusher to align the wafers thereagainst.

After the screws 54 and 56 (FIG. 1) are tightened to force the operative ends of the print wafers 80 into engagement with the control surfaces 26 and 28, and the wafers are also aligned against the pin 32, the wafers are ready to be clamped between the upper and lower clamps 98 and 88. Prior to clamping, the operating lever 70 is pushed towards the base 12 to bring the adjustable stop 66 into contact with the upper clamp 98 (as shown in FIG. 2) to compress the wafers between the clamps. While so compressed, the screws 100 are tightened to a predetermined torque to hold all the wafers in place. In the embodiment shown, the print wafers, like 80, are generally rectangular in shape having a length of 1.6 inches, a width of 1.0 inches, and a thickness of 0.020 inch. With five such wafers forming a module, the torque applied to the screws 100 was 60:2 inch ounces. Naturally, the dimensions given are merely representative of print wafers in general.

After the screws 100 (FIG. 1) have been tightened to a predetermined torque, the lever 62 which compresses the print wafers is removed, and the resulting print wafer module is inspected. To inspect the m0du1e,.the alignment bar 24 is removed permitting visual inspection of the operative ends of the wafers. The print wafer module, while still attached to the fixture plate, is then placed on an accurate flat surface so that the front surface 22 of thefixture plate 14 and the operative ends of the print wafer module are perpendicular to an accurate flat surface. If all operative edges of the print wafersdo not contact theaccurate flat surface to within 0.0003 inch, the print module is defective and the module must be reassembled after removingany defective print wafers. An air guage may also be conventionally used to measure whether or not an individual wafer contactsthe flat accurate surface. Those print wafer moduleswhich passinspection can then be removed from the jig by removing fasteners 90.

The completed print wafer modules are assembled to form a print head by the method shown in FIGS. 4, 5, and 6. An alignment fixture 1 10 is used to'align the completed print wafer modules like 112 (FIG. 4) on a support member 114 which is part of a printer (not shown) in which the modules are used.

The alignment fixture 110 has a base 116 which may be detachably secured to a stable workbench when in use. The fixture also includes a generally U shaped member having the legs 118 and 120 thereof secured to the base 116 by fasteners 122. The fixture 110 has an accurate planar surface 124 which is parallel to the faces Y126 and 128 of legs 118 and 120, respectively, and perpendicular to the top surface of base 116 as viewed in FIG. 4. A plurality of holes like 130 is drilled into the planar surface 124 so that the axes of the holes are perpendicular to the surface 124, are located on equal centers, and are all located along a straight line which is parallel to the top surface of the base 116. The holes 130 are spaced apart on centers a distance which is slightly longer than the length of the modules 112 being aligned thereby. A locating pin 132 is positioned in each of the holes 130 so that a portion of the pin projects from the surface 124. Each of the'pins 132 has a control surface which is perpendicular to the axis of its associated hole, and all the control surfaces of the pins 132 lie in a common plane which is parallel to the surface 124.

To align the print wafer modules 112 on the support member 114, the following procedure is used. The print wafer modules 112 are loosely secured to the top surface 134 of the support member 1 14 by suitable fasteners 136 as shown in FIG. 4. The top surface 134 is an accurate planar surface which is perpendicular to the flat mounting surfaces 136 and 138 of the member 114. The support member 114 is then detachably secured to the alignment fixture 110 by bringing the mounting surfaces 136 and 138 thereof into aligned engagement with the faces 126 and 128 of the fixture 110 through using suitable locating pins 140 and fasteners 142 as shown in FIG. 4. The center print wafer module 144 (identical to modules 112) is manually pushed against the control surfaces of two adjacent locating pins 132 so that the operative ends of the print wafers of the module contact the control surfaces of the pins 132. While so contacting the control surfaces, the, associated screws 136 are tightened to secure the :lower clamp 88 of the print module to the surface 134 ofithe support member 114. In the embodiment described, the screws 136 were tightened toatorque of 8 to 10 inch pounds. The print wafer modules .11 2 on each side of the center module 144 are similary aligned and fastened to the support memberltl4ylhowever, athin. insulating spacer 146 (FIG. 4) isplaced between adjacent print wafer modules prior to being fasteneditothe support member 114. Notice in FIG. 5 that adjacent print wafer modules share a common control surfaceof a pin 132 during the alignment procedure. After all the print wafer modules are secured to the supportmemberglylA, member 114 is detached from the alignment fixture and it is ready to be installed in a printer. Theprint wafer modules, after alignment,.form',a printhead for producing a line of printing. Various warm up heater plate elements like 148 and 150 maybe installed under and over the associated print modules as shown in FIG. 1 without affecting the alignment procedure. The fixture l 10 is made of anodized aluminum tool plate, and the pins 132 were made of brass.

What is claimed is:

l. A jig for assemblying a print wafer module having a plurality of print wafers sandwiched between upper and lower clamps, with each print wafer having an operative end and a rear end, and with the operative ends lying in a common plane; said jig comprising:

a base;

a fixture plate secured to said base and having accuratefirst and second planar surfaces perpendicular to each other and being joined in a common edge;

a locating pin secured to said plate near saidcommon edge and being perpendicular to said first surface and upstanding therefrom;

an alignment bar secured to said second surface and having first and second spaced control surfaces lying in a common plane which is perpendicular to said first planar surface and parallel to said. common edge;

means for detachably securing the lower clamp to said first planar surface;

first lever means supported on said fixture plate and having first and second ends for contacting :the rear ends of the print wafers to resiliently urge theoperative ends thereofagainst said first and second control surfaces as the sides of the wafers are also aligned against said pin; and

second lever means pivotally mounted on said base and having a contacting end for engaging theupper clamp to compress the wafers between said clamps while the clamps are secured together to form said module. I

2. The jig as claimed in claim 1 in which. said first lever means comprises:

a first bell crank lever pivotally mounted on said fixture plate and having said first end located on one end of the first bell crank lever;

a second bell crank lever pivotally mounted on said fixture plate and having saidsecond end located on one end of the second bell crank lever;

spring means to resiliently urge said first and second ends of said first and second bell crank levers, respectively, away from the rearends of said print wafers; and

7 8 adjustable screw means to urge the first and second said base and having said contacting end on the reends of said first and second bell crank levers rei i end th f; and spectwely mm engag emem the ends of an actuating lever for pivoting said pivotal member the prlnt wafers against the bias of said spring means. enabllng said contacting end to engage the upper 3. The jig as claimed in claim 2 in which said second clamp to compress Said wafers while the clamps are l ver mea comprises; secured together to form said module.

a pivotal member, pivotally mounted at one end on 

1. A jig for assemblying a print wafer module having a plurality of print wafers sandwiched between upper and lower clamps, with each print wafer having an operative end and a rear end, and with the operative ends lying in a common plane; said jig comprising: a base; a fixture plate secured to said base and having accurate first and second planar surfaces perpendicular to each other and being joined in a common edge; a locating pin secured to said plate near said common edge and being perpendicular to said first surface and upstanding therefrom; an alignment bar secured to said second surface and having first and second spaced control surfaces lying in a common plane which is perpendicular to said first planar surface and parallel to said common edge; means for detachably securing the lower clamp to said first planar surface; first lever means supported on said fixture plate and having first and second ends for contacting the rear ends of the print wafers to resiliently urge the operative ends thereof against said first and second control surfaces as the sides of the wafers are also aligned against said pin; and second lever means pivotally mounted on said base and having a contacting end for engaging the upper clamp to compress the wafers between said clamps while the clamps are secured together to form said module.
 2. The jig as claimed in claim 1 in which said first lever means comprises: a first bell crank lever pivotally mounted on said fixture plate and having said first end located on one end of the first bell crank lever; a second bell crank lever pivotally mounted on said fixture plate and having said second end located on one end of the second bell crank lever; spring means to resiliently urge said first and second ends of said first and second bell crank levers, respectively, away from the rear ends of said print wafers; and adjustable screw means to urge the first and second ends of said first and second bell crank levers respectively into engagement with the rear ends of the print wafers against the bias of said spring means.
 3. The jig as claimed in claim 2 in which said second lever means comprises: a pivotal member, pivotally mounted at one end on said base and having said contacting end on the remaining end thereof; and an actuating lever for pivoting said pivotal member enabling said contacting end to engage the upper clamp to compress said wafers while the clamps are secured together to form said module. 